Question:
ok...in the last two weeks I have acquired two amps.
The first (and the one that seems to work properly) is Kato's Vox AD50VT...I'm still messing around with it and i've been loving the sounds so far, but ultimately I'm a plug-and-play sort of guy.
so i bought a whimsical little amp off of Craiglist. It's an old Pacemaker 2221 20W Tube stereo amplifier converted into a guitar amp connected to a tweeter and a Celestion G12M (the "Greenback" speaker). I have attached pictures.
so now to the fun troubleshooting part:
When I played it at the guy's house I remember turning it up to get some decent volume (it's supposed to be 20W, but I have to get this thing up to 8 to get a usable practice volume), but now that I have it at home it's SUPER noisy, something I never noticed at the guy's house (same guitar)...so I decided to mess around with it and got some interesting observations to report:
1) with my Jagstang you can turn the pickups off (not volume 0, but actually "off" with on/off switches)...with the Jagstang off the amp hums REALLY loud. This also occurs with nothing plugged into the amp...thoughts on how to fix this?
2) My Strat, Jagstang, Tele and MIA bass have a pretty low volume through it. My MIM Bass (active EMG Js) is louder than any of these at the same volume setting (as i expected) and my sister's guitar (a Daisy Rock acoustic/electric with on-board pre-amp) is CONSIDERABLY louder than anything else. When I run the on-board pre-amp at about half, it is close to the volume of any of my passive instruments.
3) I said that the amp hums...this occurs with my Strat (noisy anyway), Tele (P90s = noisy), Jagstang (moreso with the single than the JB, both both are still noisy) and with MIA Jazz, but not with the MIM Jazz (active EMGs) or the Daisy Rock (on board pre-amp). I expected this to happen, but not to the extent that it did (the amp is pretty much silent)
so...here's what I'm trying to do:
1) eliminate hum, i'm particularly curious as to what causes the amp to hum with nothing plugged in.
2) increase input sensitivity. I understand that this amp is designed for active electronics (tuners, phonographs, etc) so are there resistor changes I can make to improve sensitivity without increasing noise?
I ordered the "fact sheet" on eBay today (parts list, schematics, etc. I hope) and will hopefully get it next week. Please ask me any questions you might have about it that will help us reach a solution to my problems (or if you're just curious about something)
Answer:
The hum sounds like it could be a grounding issue at your house. Does it have 3 prong plug on it? Get a ground adapter and see if that takes care of it.
Answer:
Originally Posted by Dave H The hum sounds like it could be a grounding issue at your house. Does it have 3 prong plug on it? Get a ground adapter and see if that takes care of it. the amp does have a ground plug on it. when i open it up i'm going to check and see if/how it is grounded as i was also thinking the hum could be a grounding issue.
Answer:
When the electrolytic filter caps in the power supply get old and dry up, they lose their capacitance - which is what filters AC hum out of the DC votage. That looks like its from the 60's, maybe early 70's, either way if they are original, they very likely need replacing. Other than that, it could be anything, especially since someone messed around with it.
Have you tried the other inputs? Try plugging it into the "mag phono" input for more gain, or the "ceramic phono" for less than that. Both of those were for the direct output of a phono cartridge - not preamped, so they should have a good amount of gain, maybe too much..? Mic input, same way, a tape input would be for a line level. I can't see what the marking is for the input you have it in.
When you get the schemo, post it. One thing I've heard about guitar amps in general - they run the tubes at substantially higher voltages than they were originally designed for, which was hi-fi reproduction. I think typically 100 volts or better over the spec. That unit would have been designed for hi-fi stuff from the ground up - preamp, power tube circuitry, output tranny. If its got 4 6V6's like I think I see, and its only rated at 20 watts, it may even be a true class a...?
I would also ditch the tweeter, its only going to reinforce highs that you don't need, causing harshness, that's also where a lot of hissy noise is.
Answer:
The tube complement is
4x6V6 (power tubes I assume)
1x6CA4 (rectifier I think)
1x12AU7 (power-amp driver?)
4x12AX7 (pre-amp tubes)
right now it's plugged into the "aux" input. there's also a Hi-Z out, tuner, ceramic and magnetic phono jacks and a tape head jack. Unfortunately the current cable only allows me to plug into the Hi-z, tuner and aux jacks. it's not quite long enough to reach anywhere else.
I'm going to try disconnecting the tweeter first. I suspect that this will kill the high end buzzing, but I'm certain it won't relieve it over all (it actually sounded really good with the acoustic guitar). I'll be getting the schemo in the mail and I don't have a scanner, but maybe i can find someone with one, or take pictures and post them (i've done that successfully for school projects)
Answer:
Hi Z means high impedance, probably intended for a hi Z microphone. I would bet that one will have higher gain than the aux. Try it in there. Aux I'm pretty sure would be for a line level input - low gain, that's the wrong one to use for guitar. That 6CA4 is a rectifier tube, a small one too, hmmm. I guess the hi Z out is for slaving another amp.
Oh, I just noticed that's a stereo....You are only running 1/2 of it.... My guess is that at least on the aux and tuner (line level) its only using one preamp tube stage - not enuf for a passive guitar signal.
You might also try using a buffering pedal, and run a Y cord out, then run each one of those into 2 different passive inputs, like ceramic, mag, or Hi Z. That would add gain at the power tube stage., since it would get hit with the output from 2 channels in parallel. Years ago, guys used to jumper the 2 channels on Fender (non-reverb) amps for a bit more gain.
Of course, an obvious solution is to use a preamp in front of it. That probably would be a good amp to use with a modeler.
If you are into hacking on it, you could rewire it a bit and cascade an extra preamp stage or two - there's 8 available in there with the 4 12AX7's.
You might do a search, and see if you could bridge the output of the two channels into one, and then jumper the L and R inputs.
Also - since that has the tube rectifier, you should go ahead and discharge the big caps if you are going to mess with it - it may not have any bleed-off circuitry to get rid of the charge in 'em like SS rectifier'd tube amps.
Answer:
one of the problems i'm going to have with anything in terms of using what i've got to bridge inputs or run dual inputs is that the inputs are RCA jacks. i hadn't considered this though...what would it be like if i wired up anoter RCA jack at the guitar input and ran it to the inputs on the amp? would this serve the same purpose
Answer:
Yeah, where the guitar input is, put 2 RCA jacks, and just jumper to whatever.
Here's another thing to add gain. Take out one of the 12AX7's (powered off) that's not used by whatever channel you use. Put that into the spot where the 12AU7 is, that should yield a significant increase in gain by driving the power tubes harder. Being a stereo amp, I'm guessing that it is a real class A; otherwise, it would need 2 phase inverter tubes, which I don't think yours has.... The fact that you said the 12AU7 was a tube driver leads me to believe that. I think real class A was used in hi-fi tube stuff pretty often. I've read that in guitar amps there is no advantage to real class A, that you would never be able to tell the difference between it and A/B, but in hi-fi stuff, yes.
Just occured to me - Probably why Vox or whoever first (mis) used that for guitar amps may have been due to the fact that in the 50's and 60's, just about everything was tube, and that it would have been common for people to have a bit of knowledge and conception that class A was better, 'cause that's what all the higher end audio stuff may have been... So one of the Vox marketing guys asked around and found out that at low volumes, the way they are cathode biased, they actually do function as Class A, morphing into A/B as the volume is raised.
Answer:
i have no idea if the 12au7 is a tube driver. i just have read that it's a common application for 12au7s in power amp structures (and i figured with 4-12ax7s, why would you include a 12au7 in the preamp).
i really can't wait to get the schematic next week...so awesome!